Welding metals with beams of high energy density (heat source), typically laser beams, is used in a wide range of metal welding applications since the welding process is clean, has a high degree of freedom, and can minimize thermal strains. Particularly, the laser beam welding process for metals finds growing use in the automobile industry or the like as a composite machining system composed of a laser oscillator and a numerically controlled apparatus or robot.
Automobiles employ a number of corrosion-resistant galvanized steel sheets as a highly durable material in their frames and other components. There is a demand for welding galvanized steel sheets with a laser beam.
FIG. 2 of the accompanying drawings shows a conventional process of welding galvanized steel sheets with a laser beam. A galvanized steel sheet 1 has plated zinc layers 2, 3 deposited on its opposite surfaces, and another galvanized steel sheet 4 also has plated zinc layers 5, 6 deposited on its opposite surfaces. When a laser beam 8 is applied to the galvanized steel sheets 1, 4 to weld them, a weld bead 10 is formed as a result of a weld pass.
Steel has a melting point of about 1545.degree. C. and a boiling point of about 2754.degree. C., whereas zinc has a melting point of 420.degree. C. and a boiling point of 903.degree. C. Because of the heat produced by the high-density energy of the applied laser beam, the zinc with a lower melting point is violently evaporated into a gas, thereby forming porosity (cavities) 11a, 11b, 11c in the weld bead 10. While only the surface porosity is shown, porosity is also formed within the weld bead 10. As a result, the weld bead 10 has irregular width dimensions, and the welded joint is rendered brittle. For the reasons described above, it has been infeasible to weld galvanized steel sheets with a laser beam.